One aim of the invention is to provide for an improved intruder alarm system in which the status of each one of a relatively large number of sensors, distributed for example throughout a building, is individually identifiable. Such "individual detector identification" (IDI) systems are already known. In a conventional IDI system the various sensors are connected to a central control unit in a loop and each one returns a signal indicative of its state when addressed in turn by a signal passed from the preceding sensor in the loop. Since each sensor has to have its own processing and signalling electronics this arrangement is relatively expensive, and can also cause problems with the amount of trunked wiring and multiplicity of connections involved. A more particular aim of the present invention is therefore to achieve an IDI capability more cost-effectively than with the conventional looped system.
Another aim of the invention is to provide a system capable of handling sensor outputs in analogue voltaic form. Analogue sensors clearly have the advantage over binary output sensors of being able to return data concerning a range of sensed conditions. More than this, however, a system which is capable of discriminating a range of different output voltage levels from a given sensor--as opposed to simply the presence or absence of a voltage, or a voltage above or below a predetermined threshold--can be used to advantage for diagnostic purposes, e.g. for detecting and compensating for drifts in output voltage due to ageing or contamination of sensor components, or for identifying other faults or failures which result in off-normal outputs. The advantages of fault-diagnosis apply equally well to binary sensors having an identifiable "normal" output voltage as they do to true analogue sensors. It is not, therefore, an essential feature of a system according to the invention that it actually employs analogue sensors, only that it is capable of so doing by virtue of its ability to transmit signals representing a range of sensor output voltages.
The present invention accordingly resides in a data acquisition system comprising: a plurality of distributed sensors each one of which is adapted to provide a voltage output indicative of a value or condition sensed thereby; a plurality of nodal units to which the outputs of respective sets of said sensors are connected; and a central unit adapted to receive data from said sets of sensors in response to its repetitive interrogation, in turn, of the respective said nodal units to which the sets of sensors are connected; each said nodal unit being adapted repetitively to derive in respect of each said sensor in the set connected thereto a pulse signal the width of which represents the voltage level of the respective sensor output, and to transmit the corresponding set of pulse signals in turn to the central unit when interrogated thereby.
In a system according to the invention, therefore, the output of each individual sensor is identifiable from its order position in the set of pulse signals transmitted by the respectively interrogated nodal unit, while the necessary processing and signalling capability is effectively shared between the members of each set of sensors at the respective such unit. By the use of suitable multiplexing circuits in the nodal units (also referred to hereinafter as "concentrators") the overall processing time required to complete a scan of the status of a given number of sensors can be considerably reduced in comparison with a conventional looped IDI system as described above. The arrangement of nodal units can also simplify the wiring and connections required. The pulse-width modulation regime incorporated at the nodal units means that analogue data can be reliably transmitted to, and decoded at, the central unit.
In a preferred embodiment it is arranged that the output voltage of each sensor, at each scan, is integrated at the respective nodal unit over a period corresponding to one cycle of the local mains supply, thus to eliminate the effects of any mains hum on the sensor outputs. Other preferred nodal unit features include the ability to switch power to its connected sensors only during those periods when their outputs are being scanned, and to phase its transmission of the pulse-width modulated signals in relation to its scanning of the sensor outputs and the transmission of the other nodal units to achieve a minimum cycling time.